1. Technical Field of the Invention
This invention relates to electromagnetic interference filters, and in particular to a bulkhead mounted, center feed EMI filter for low frequency, high current applications, utilizing a non-ferroelectric dielectric material such as NPO/COG.
2. Background Art
Electromagnetic interference (EMI) is created from everyday sources such as lightning, rain and even strong winds. Additionally there are innumerable sources of man made EMI typically created and radiated by televisions, power transmission lines, ignition systems, fluorescent lightning, radar transmissions, electric car chargers, and computing devices. These sources of EMI radiation challenge the equipment, designers and engineers to find a solution to keep electronic signals coming to equipment clean and usable, isolated from the negative effects of the ever present EMI.
EMI filters, electronic devices having suitable capacitive and inductive characteristics for resisting or shorting out the onerous EMI, are commonly installed in electronic circuits to achieve this goal. However, current EMI filters and the practice of using them has several problems including: manufacturing defects in the filters resulting in poor performance; incomplete product specifications leaving users with insufficient information about the filters and how to use them; difficulty in selecting the correct device for the application; and improper installation of the filter for best result in the circuit of interest.
Filters for the application of interest here are commonly fabricated by prior art methods and typically consist of a discoidal capacitor, feed-thru filter in a bulk head mount configuration that is placed in a signal path to shunt any electromagnetic interference in higher frequency ranges to the ground via at least one capacitive component.
Feed-thru filters of this type consist of either a capacitor (C-only) or a combination of capacitive and inductive elements arranged in classic filter configurations (LC, Pi, or T). Each of these capacitors fits a particular application requirement and proper selection is critical. The most economical solution is to select the filter with the fewest internal parts that otherwise achieves the desired filtering effect.
C-only filters, filters that consist solely of capacitive elements, are best suited for filtering high frequency signals on lines with very high impedance. The attenuation of these devices increases in steps of 20 dB per decade from the filter's cutoff frequency up to the frequency where they reach an attenuation of at least 60 dB.
The LC type filter is best suited for applications where there are large differences between line and load impedances. These devices consist of a capacitive element, in the same manner as the C-only filter, with the addition of an inductive element connected in series with the capacitor between the input and output terminals. Usually, it is best to install the filter so that the inductive element faces the lower impedance terminal. With respect to the conventional packaging of discoidal capacitor type filters, this means that in some applications it is desirable to have the capacitive element close to the threaded or screw-neck header end of the filter package, while in other cases the reverse is desirable, with the inductive element located on the threaded or screw-neck end.
Unlike conventional leaded capacitors, the discoidal capacitor's co-axial configuration provides two unique advantages. It prevents radiation present at the input end from coupling directly to the capacitor output. This construction also has inherently low self-inductance and the combination provides excellent shunting of EMI at frequencies approaching 1 GHz. The addition of inductive elements (wire wound coils, toroids or beads) in series with the capacitor increases the impedance of the line, making the filter even more effective.
Pi filters consist of three elements. A series inductive element is positioned between two capacitors which are shunt connected to the electrical ground plane. Pi filters are best suited for applications where the input and the output impedances are of similar value and high levels of attenuation are required. These filters typically increase attenuation by 60 dB per decade from the filter cutoff frequency to the frequency where the filter exhibits an attenuation of at least 80 dB.
The T filter is also a three-element device, but this time there are two series inductors connected between the input and output terminals on each side of a single capacitor which is shunt connected to the ground plane. They perform in much the same manner as a Pi filter, increasing attenuation in steps of 60 dB per decade from the cutoff frequency to the frequency where the attenuation is at least 60 dB. This filter type is selected when both the input and output impedances are low.
Internally the most complicated device, the LL filter consists of two feed through capacitors connected between line and ground interspersed with two inductors connected in series between the input and output terminals. These filters increase in attenuation in steps of 80 dB per decade from the cutoff frequency to the frequency where the attenuation is at least 80 dB.
Today, most center through-feed, metal housing, bulkhead or through hole mounted, EMI filters for low frequency, high current applications employ at least one discoidal capacitor element, and commonly use X7R ceramic formulations for the capacitor dielectric. It is cost effective and has an adequate dielectric constant at normal operating temperatures. However, this ferroelectric material has certain inherit disadvantages. Over time this dielectric will experience capacitance loss due to intercrystalline aging. This capacitance loss occurs logarithmically when temperatures go below the Curie point of the formulation. The applied voltage, AC or DC, also affects capacitance when the voltage level is low compared to the rated voltage of the device. When this happens, a polarizing effect takes place that can drop the capacitance value of the device by as much as fifty percent of the original value. The X7R dielectric also exhibits a relatively large change in capacitance with operating temperature. The capacitance drops as much as 10% from the original value at high end, +85 C., and low end, −55 C., temperatures.
Another undesirable attribute of the X7R dielectric, particularly in AC applications, is its loss characteristics or dissipation factor. The dissipation factor loss tangent is the quotient of the active and reactive components of the impedance. DF is the measurement of dielectric losses and is dependent on temperature and frequency. Dielectric loss is the result of the changing polarization of the dielectric caused by alternating fields. They are transformed into oscillations and thus produce frictional heat. Typically the DF is as high as 0.02 absolute at 1 kHz to over 0.04 absolute at 100 kHz, at +25 C. This loss characteristic is directly related to the self-generated heat rise inside the device during AC operation. This heat rise must be added to the ambient operating temperature to have an accurate indication of the filter's actual temperature during operation. In this case, the EMI filter may be running at +160 C. internal temperature when it is believed to be operating at +125 C.
The above situation causes serious problems since it places stresses on the dielectric that may not have been expected or understood by the equipment designer. The first of these is mechanical stress. In the winter or at high altitude or relatively cooler environmental temperatures, the differences in the thermal coefficients of expansion between the ceramic capacitor, the metal housing and the center conductor will be understood to be quite large. This puts unnecessary stresses on the fragile ceramic layers than can lead to cracking and delamination. The breakdown strength of the dielectric layers is also affected by temperature and will in time lead to a premature failure of the device in the form of an electric short. Unfortunately, this root cause of failure, cracks in the ceramic dielectric layers, is frequently misdiagnosed during failure analysis.
Improper soldering of filter leads is also a problem that often occurs with feed-thru filters. A soldering iron held too long during installation may cause the ceramic capacitor to develop microcracks in the dielectric layers since heat is conducted down the center conductor to the ceramic capacitor. Over time these tiny cracks develop debris and contaminants and in conjunction with the applied electric field, develop conductive paths leading to ever increasing leakage current until an electrical shortage occurs. Also the center conductor of the ceramic capacitor may actually be unsoldered, removing the capacitor from the filter circuit completely, which will drastically affect the desired attenuation of the filter.
Other commercially available impedance devices, typically surface mounted and used for lower current applications, include chip and miniature discoidal capacitors, which are known to be available with different dielectrics. Multi-Layer Chip Capacitors (MLCC), and miniature discoidal capacitors, for example, may use any of NPO/COG, X7R and Y5V dielectrics. The choice of dielectric material is usually determined by the required capacitance-temperature stability. Class I capacitors or temperature compensating capacitors have predictable temperature coefficients and in general do not have an aging characteristic.
The most popular class 1 multi-layer ceramic chip capacitors are COG (NPO) temperature compensating capacitors. EIA class 2 capacitors typically are based on the chemistry of barium titanate and provide a wide range of capacitance values and temperature stability. The most commonly used class 2 dielectrics are X7R and Y5V. MLCC class 1 NPO/COG dielectric capacitors are typically used in such applications as coupling, LC networks, high frequency EMI, band pass, lowpass filter, cellular phone, high frequency amplifier, thin film, thick film, hybrid circuits and modules, LTCC, Blue tooth circuit board, Cordless Phone, Wireless LAN, PDA, and RF modules. However, the applicant is not aware of any low frequency, high current, center feed, metal packaged EMI filters for bulkhead or through hole mounting that utilize a NPO/COG or equivalent material for the discoidal capacitor dielectric elements.
It is clear that an improved, electromagnetic interference filter of the discoidal capacitor, center feed-through type, configured in a metal package for bulkhead or through-hole mounting, expressly for low frequency, high current applications in the range of DC to 3000 hertz and up to 40 amperes current, and further up to 100 KHertz at lower current values, that can overcome the mechanical and thermodynamic stresses that occur in common bulkhead installations and maintain a relatively uniform degree of performance over a temperature range of −55 to +125 C. with extended life expectancy, would be useful.